The present invention relates to an exposure method used for transferring a mask pattern onto a photosensitive substrate in lithography processes to produce, for example, semiconductor devices, liquid-crystal display devices, or thin-film magnetic heads. The present invention also relates to a device producing method using the exposure method.
Semiconductor devices or other similar devices are produced by using one-shot exposure type projection exposure apparatuses (e.g. steppers) in which a pattern formed on a reticle as a mask is projected onto a photoresist-coated wafer (or a glass plate or the like) through a projection optical system, or scanning exposure type projection exposure apparatuses such as the step-and-scan type. The degree of integration of semiconductor devices is becoming increasingly higher, and patterns to be formed are becoming even finer. Consequently, projection exposure apparatuses are required to provide even higher resolution.
In general, resolution is proportional to the wavelength of exposure light and inversely proportional to the numerical aperture (NA) of the projection optical system. Therefore, straightforward methods of improving the resolution are to use exposure light of shorter wavelength and to increase the numerical aperture of the projection optical system. Accordingly, excimer laser light, e.g. KrF excimer laser light (wavelength: 248 nanometers) or ArF excimer laser light (wavelength: 193 nanometers), is also used as exposure light these days. However, because depth of focus is inversely proportional to the square of the numerical aperture, the depth of focus becomes excessively shallow if the numerical aperture is merely increased. In a case where a projection optical system with a large numerical aperture is used, when defocus occurs, the pattern length varies to a considerable extent owing to the effect of wavefront aberration, although the reduction in pattern length is not remarkable at the best focus position.
To improve resolution, it has been proposed that illumination optical systems should adopt annular zone illumination which uses an annular zone-shaped aperture stop, or modified illumination which uses an aperture stop consisting essentially of a plurality of small decentered apertures. Accordingly, attempts have recently been made to combine a projection optical system having a relatively small numerical aperture and the annular zone illumination to transfer fine patterns. Even when such a technique is used, the conventional practice is to transfer one pattern by a single exposure operation.
To transfer a fine periodic pattern having a small pitch, for example, there has been proposed a technique whereby the periodic original-plate pattern is split into a plurality of separate patterns each having a pitch larger than that of the original pattern, and the images of the separate patterns are superimposed on one another by multiple exposure.
Among the conventional techniques as described above, the technique whereby a projection optical system with a small numerical aperture (NA) and the annular zone illumination are combined to transfer a fine pattern requires a large amount of light exposure because only a part of diffracted light generated from the fine pattern can pass through the projection optical system because of the small numerical aperture. In a case where the length-to-width ratio of the fine pattern is widely different from 1:1, for example, and where the pattern is sufficiently long in the longer dimension, so that the dependence of the image intensity on the numerical aperture in the longer dimension is small, if an amount of light exposure with which the width (line width) in the shorter dimension of the pattern is correctly formed is applied, over-exposure occurs in the longer dimension of the pattern. If a positive resist is used in such a case, the pattern length in the longer dimension becomes undesirably shortened.
It is difficult to correct the change in pattern length by adjusting the length in the longer dimension of the original-plate pattern used for exposure. The reason for this is as follows. The amount of correction for the change in pattern length varies according to the periodic structure in the longer dimension of the original-plate pattern. Therefore, it is necessary to determine an amount of correction by executing an extensive case sorting operation. Accordingly, it is actually difficult to correct the change in pattern length by this method.
In the conventional method in which a fine periodic pattern is split into a plurality of separate patterns, and multiple exposure is carried out for the separate patterns, no particular consideration is given to the difference in dimensional errors between the longer and shorter dimensions of the pattern.